错误 400 表示您的请求中的 JSON 无效:

Error 400 means an Invalid JSON in your request:

检查 JSON 消息的格式是否正确并包含有效字段(例如，确保传入正确的数据类型).

Check that the JSON message is properly formatted and contains valid fields (for instance, making sure the right data type is passed in).

在您的 sendRequest 中，您错过了 "news" 和 "data",)/code> 和右括号 (}):

In your sendRequest, you missed a comma (,) between "news" and "data" and a closing bracket (}):

String postJsonData = "{"to": "/topics/news""data": {"message": "This is a Firebase Cloud Messaging Topic Message!"}";

看起来像这样:

{"to": "/topics/news/""data":{"message":"...."}

应该是:

String postJsonData = "{"to": "/topics/news", "data": {"message": "This is a Firebase Cloud Messaging Topic Message!"}}";

以便 JSON 结构正确:

So that the JSON structure would be correct:

{"to": "/topics/news/",
 "data":{"message":"..."}
}